I got that information from the Gold Anti-Trust Action Committee. They have email updates periodically and have watched the decline in the American SDR's lately.
I don't know about SDR's being tied to a basket of currencies, but if they are, it is probably all tied back to gold. An SDR is valued as 1/35 of an ounce of gold. The value of currencies changes by the minute, therefore a basket of currencies as a unit of value would have to be adjusted itself by the minute to have any real meaning as a unit of value, and would be a nightmare as a mechanism for setting the value of debt or assets.
See:
www.gata.org
www.goldensextant.com
www.kitco.com then click on forum for gold discussions
www.lemetropolecafe.com